Forest tent caterpillars in Minnesota

The forest tent caterpillar, Malacosoma disstria, occurs
throughout most of the United States and Canada wherever hardwood
trees are found. It periodically infests aspen and many other tree
species over large areas of northern Minnesota. The caterpillars are
commonly, but mistakenly, called 'armyworms'. Groups of
these caterpillars can defoliate trees, potentially injuring them.
They are often considered a nuisance when they are found around human
dwellings or on roads.

Life cycle and description

Forest tent caterpillar larvae emerge from egg masses in early to
mid-May, about the same time aspen leaves begin to open. Caterpillars
feed actively on aspen and other broadleaf trees for five to six
weeks. Despite its name, the forest tent caterpillar does not make a
true silken tent. However, the larvae do spin an inconspicuous silken
mat where caterpillars congregate on the trunk and branches.

In June, older larvae become restless and move around trees and other
vegetation to find food. Significant damage to nearby plants can occur
at this time. Near the end of June, full-grown caterpillars wander
from where they have been feeding to search for protected places to
spin silky cocoons (to pupate). Full-grown caterpillars are about two
inches long, mostly blue and black, with a row of white, footprint
shaped markings on their backs, and many hairs along the edge of the
body.

Adult moths emerge from cocoons about two weeks later in mid-July.
These tan moths are nocturnal and are attracted to lights at night.
Adults live for about five days. During this time they deposit 100 to
350 eggs in gray cylindrical masses surrounding small twigs. The eggs
overwinter and larvae hatch from them during the next spring. There is
only one generation per year.

In Minnesota, the number of forest tent caterpillars changes in
relatively predictable cycles. At the beginning of this cycle, forest
tent caterpillars can be difficult to find. Over a period of eight to
thirteen years, their populations start to increase until they reach
tremendously large numbers. These outbreaks usually last for about
three to four years in Minnesota, although in southern areas of the
United States outbreaks have been known to last as long as nine years.

Finally, natural controls, including cold or damp spring or early
summer weather, starvation, and viral disease, can cause populations
to crash to very low numbers. Populations are also significantly
reduced by wasps and flies that parasitize eggs, larvae, and pupae.
The most conspicuous of these parasitic insects is a large gray fly,
Sarcophaga aldrichi, which is often a nuisance to humans. This
fly is native to Minnesota. Their numbers increase in response to
forest tent caterpillar outbreaks.

Food plants

Forest tent caterpillars can defoliate a large number of broadleaf
trees and plants. In Minnesota, quaking aspens, Populus
tremuloides, are most frequently attacked. Other highly preferred
tree species in Minnesota include balsam poplar, basswood, oaks,
ashes, birches, alder, and fruit trees. When trees are defoliated,
forest tent caterpillars may also damage other nearby plants. Damage
has been found on vegetables, fruit trees and other small fruits, and
nursery crops. This caterpillar rarely feeds on red maple and
conifers, such as pine and spruce.

Damage

Trees that are fed upon by forest tent caterpillars are rarely killed
by these insects because following complete defoliation, deciduous
trees are able to produce another set of leaves during the same
season. The main impact of forest tent caterpillar feeding on
deciduous trees is a reduction in the rate of growth. Vigorously
growing trees can tolerate up to two or even three consecutive years
of heavy defoliation without suffering serious damage or mortality. If
a prolonged defoliation cycle occurs, (four or more years), moderate
to heavily defoliated trees may experience a reduction in growth,
suffer branch dieback and could eventually be killed. A general rule
of thumb is that complete defoliation can occur when an average of
nineteen egg masses are found on an aspen tree that has a six inch
diameter trunk at about four and half feet. When trees become
stressed, such as during a drought, they are less tolerant of
defoliation. A stressed tree can be injured or even killed in a much
shorter time period than an unstressed tree. Protecting high value
trees would be appropriate during forest tent caterpillar outbreaks
when trees are under moisture stress.

Nuisance behavior

When mature larvae wander to find a place to pupate, their search
often takes them to nearby buildings and yards. They do not bite
people or harm animals or property, but they can be very annoying when
large numbers are found crawling or clustering on building walls,
lawns, and sidewalks. They also become a nuisance when they build
cocoons on the sides of buildings. These cocoons can be very difficult
to remove. These activities are sometimes more upsetting to people
than any feeding damage that they inflict on trees. When forest tent
caterpillars cross roads and streets and are crushed, they can cause
surfaces to be greasy and slippery.

The large gray parasitic fly, Sarcophaga aldrichi, often
becomes very abundant during forest tent caterpillar outbreaks and is
itself a nuisance. Known as the friendly fly, this native parasite
lands on any object, including people. It is also sometimes referred
to as the government fly because of the mistaken belief that a
government agency has released it (however, this is a naturally
occurring fly). This fly does not bite and is harmless but it moves
very deliberately and often is difficult to discourage. This insect is
very important for ending a forest tent caterpillar outbreak
naturally. The most practical solution is to tolerate this beneficial
insect until its numbers also naturally decline.

Management—home landscape

In the home landscape, simple physical procedures (mechanical control)
by the homeowner can be carried out to help manage the forest tent
caterpillar. One management procedure is to remove and destroy
overwintering egg masses from branches of small trees before eggs
start to hatch in the spring. Furthermore, caterpillars and cocoons
can be brushed off houses, picnic tables, or decks with a stiff broom
or brush or knocked down with a spray of water. Be careful not to
crush too many caterpillars; they can smear and leave marks on some
paints.

Another management tool available to the homeowner involves chemical
treatment with insecticides. Insecticides should be sprayed when
caterpillars are small and easy to manage. Larger larvae are more
difficult to kill and can continue to heavily defoliate trees before
some insecticides take effect. An effective larval insecticide is
Bacillus thuringiensis var. kurstaki (also referred to as
BT), a microbial insecticide derived from a bacterium. BT offers
effective control and conserves beneficial insects. Other insecticides
available to homeowners that conserve beneficial insects are
insecticidal soap, spinosad (Conserve), and azadirachtin (Azatin).
Additional insecticides available to homeowners include carbaryl
(Sevin), malathion, acephate (Orthene), and permethrin. (See table.)

In addition to physical controls, an insecticide may give some relief
from migrating larvae. Spray a product labeled for use around the
outside of buildings (such as permethrin) on the exterior of homes.
Use all insecticides carefully and judiciously. Do not overspray or
repeat treatments more often than is allowed by the label.

Active ingredients are often listed in fine print so check the label
carefully. Common examples of trade names are given strictly for
convenience. Use of trade names does not imply endorsement.

Follow all label directions carefully. Be sure the insecticide is
labeled for the plant that is intended to be treated. Insecticides
should never be sprayed on surfaces which people routinely contact or
on hard surfaces that can accelerate insecticide runoff into water.

The availability and recommended use of specific insecticides may
change at any time. If suggestions in this publication differ from
recommended uses on a label, the label is the final authority on how
you may legally use that insecticide.

Management—wood lot

In wood lots, many animals, such as birds, rodents, and even bears,
eat forest tent caterpillar larvae. People need to be more tolerant of
these pests, unless defoliation is severe and prolonged enough to
cause branch and tree mortality. In areas experiencing high levels of
forest tent caterpillar defoliation for a number of years, aerial
spraying of an insecticide may be considered as a management option.

In wood lots, resort areas, and campsites where a relatively large
acreage may need treatment, application of the spray by aircraft is
the most rapid, effective, and economical method. Spraying should not
be conducted when breezes threaten to drift the insecticide over open
water or other sensitive areas. When a recreational or residential
area is sprayed, an additional strip about 400 feet wide adjacent to
the area should also be treated. This barrier strip will serve to
absorb migrating caterpillars.

As is the case in the home landscape, the insecticide BT is preferred
since it does not harm people, birds, or beneficial insects and is
generally considered first in aerial spray programs. Some insecticides
are not registered for use by aerial spraying, so consult the
insecticide label. Dimilin (diflubenzuron) should not be used near
wetlands or water as the insecticide is a chitinase inhibitor and may
affect aquatic insects and other arthropods.